Comparison of Continuous Flow Analysis Techniques A Laboratory Exercise C. L. M. Stults, Paul R. Kraus, S. K. Ratanathanawongs, Chas. J. Patton, and S. R. Crouch Michigan State University, East Lansing, MI 48824 Continuous flow analysis (CFA) is the technique by which a sample undergoes treatment as i t is carried by a flowing stream through a manifold made of interconnected tubing. This technique can he subdivided into two major areas: aiiseemented and nonseemented continuous flow analysis. I n t b i former technique,air segments are introduced into the analytical stream a t regular intervals so that the stream is divided into a train of identical segments. The sample is introduced onto the manifold hy aspiration for a defined period of time. In each segment nearly complete mixing takes place so that the signal obtained a t the output has a rectanrmlar shane similar to what would he exoected in the ideal G s e of a plug-shaped sample. Skeggs inkoduced airseemented continuous flow analvsis (ASCFA) as a novel approach t o automated chemical analysis in 1957 (1). For many years, air segmentation was thought to he essentialfor successfulCFA (2).However, in the mid-19708, Stewart and co-workers (3) in the United States and Ruzicka and Hansen (4) in Denmark demonstrated that CFA could he a useful technique without the aid of air segmentation. Flow (FIA). iniection analvsis " , ., also known as nonseemented continuous flow analysis, incorporates three principles: sample iuiection. controlled disoersion. and reoroducihle timine. ~ i underlying e theory add appli'cations 02 these characteristics are discussed in two recent monographs (5,6). There are advantages and disadvantages for each of the two techniques with respect to complexity of the apparatus, sample and reagent c&sumption; mixing efficiency, and dispersion. Since hoth ASCFA and FIA have found numerous applications in industrial, clinical, chemical, and academic research laboratories, i t is important for students a t the eraduate and undereraduate levels to understand the merss of each technique. There have been cooperative efforts between industry and academia in the past t o introduce ASCFA t o undergraduates (7, 8). This was necessary because of the hieh cost of the apparatus. We have designed .and construct& a relatively inexpensive, single-channel, miniature CFA svstem (9) that can he used for ASCFA as well a s FIA. withihis, or a similar system, direct comparison of the two techniques can he made. Previously reported FIA experiments havi dealt exclusively with that particular approach to CFA (10-13). This lahoratorv exercise consists of three experiments. first allows tge students to compare the dispekion characteristics of the two techniques. The determinations of chloride and nitrite are used to demonstrate the practical differences between ASCFA and FIA. The experiments detailed here were a n outgrowth of the work reported by Patton e t al. (9.14.15). They have been part of a graduate-level analytical chemistry co&e in our department for the last four years. The chloride and nitrite determinations have also been incorporated into a n upper level undergraduate instrumental analysis course. Two laboratory sessions of three hours each &e required for completion of the experiments. ~~~
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Experimental Apparatus A few basic pieces of equipment are necessary for the set of experiments described here. First, a peristaltic pump that has at
1060
Journal of Chemical Education
least six channels is required. It should deliver a relatively pulse-free flow. Mild pulsations can be dampened by incorporating a length of soft silicone tubine (-1.5 m) between the pump and the beginning of the manifold. A &nually operated injection Galve that c& be configured for variation of sample size is adequate. The detection system may consist of a suitable spectrophotometer equipped with a flow-throughcell (volume less than 20 pL) and connected to a chart recorder. Several different configurationsof the system components are required; these are discussed individually in the Exercises. For the ASCFA experiments some method of debuhbling the stream prior to detect& is required. This may be done physically (2) or electronically (14, 16, 17). Physical dehubbling is aceomdished bv insertion of a tee . iust . orior to detection. Electronic bubble gating of the signal is preferred since it allows more rapid attainment of steady-state conditions (lower interaction between samples) and maintains the integrity of the analytical stream (16). This method requires a very small volume flowcell that is completely filled with liquid between air segments. In our laboratory, we use a commercially available flow cell with a path length of 1.0 mand an internal volume of approximately 2 pL (Gamma Enterprises, Mt. Vernon, NY). We chose to use the electronic bubble gate developed in our laboratory (14). The bubble gate and the other components of our system are shared between the teaching and research laboratories and therefore were conveniently available. More specifically,our system was composed of the following: a 12-channelperistaltic pump (Ismatec, model IP-12) with flow-rated tubing (Technicon);a manually operated six-port sample injection valve (Rheodyne Type 50) (for the FIA experiments); a miniaturized flow-through filter calorimeter (designed and constructed in this laboratory (15));and, astrip chart recorder (Heath). Manifolds for the FIA and ASCFA systems were constructed as previously described (9). For the FIA system, O.Bmm-i.d. Teflon tubing was used in the manifolds. Adjustable sample volumes were obtained by varying the time that the valve was in the "inject" position. In this position, sample was continuously pumped through the manifold. In the "fill" position, carrier was pumped through the manifold. The single bead string reactor (SBSR) was composed of 0.5-mm-diameter nonporous glass beads in 0.8-mm Teflon tubing. The tubing was pinched at the end with needle-nose pliers to prevent the beads from escaping. The ASCFA manifold was made with 1.0-mm4.d.tubing. All mixing coils had a coil diameter of approximately 1.5 an. It should be noted that, because of our equipmentsharing arrangement, it was possible to have two station-one for ASCFA and the other for FIA. Reagents All solutionswere prepared withdistilled water (DW)and filtered as necessary. Continuous flow experiments are usually performed with a surfactant added to the solutions to allow wetting of the tubing. Therefore, Brij-35 was added to all solutions in the proportion of 1 mL to 1L of solution. All solutions were made prior to the beginning of the laboratory sessions. Disoersion Solutions. Borate buffer (2 L.oH oreoared .. 9.5). was . . as drsrrfbed elsewhere (123). A stock solution of phenol red ( 1 m M )was prrporrd by nddingO.l gofthedye ttra230-mLvolun~etristlaskand diluting to thp mark with0001 M . NaOH. The wurkingaolution was composed of 5 mL of the stock solution diluted to 500 mL with the borate buffer. Chloride Determination Solutions. A 1000-pg mL-' chloride stock solution was made by transferring 0.412 g NaCl to a 250-mL volumetric flask and dilutine to the mark with DW. Chloride standards (500mL of each) with ¢rations of 5.15.25.35. and 45 .ue" ml: ;ere prepared hy approprrate d~lution
